1. Field of the Invention
The present invention relates to a structure and a method for heating tanks containing a bath of molten metal, in particular galvanising tanks for steel wires in which the wires are moved in the form of a sheet in a substantially continuous manner in the direction of their length through a bath of molten zinc.
2. Description of the Related Art
These galvanising tanks may have a metallic crucible or a ceramic crucible.
Different methods of heating these tanks are currently in use.
If tanks with crucibles made of steel are used, it is essential that the temperature of the latter is limited to 450.degree. C., given that above this temperature the iron in the crucible is easily taken into solution, with the result that the corrosion of the crucible by the molten zinc contained in it is accelerated.
Because of this, the method of heating such tanks should be designed so that the temperature of the crucible walls of the said tanks at no time exceeds the critical temperature of 450.degree. C.
In this connection, there exist different heating methods, namely heating by high speed circulation of hot fumes, heating of the side walls by radiation making use of burners described as "flat flame burners", heating by electrical resistors placed along the walls and above the tank, and lastly induction heating.
For tanks with ceramic crucibles, these being formed by components made of a refractory material, even of cast concrete, there is consequently no limitation on the temperature, unlike in the case of tanks with steel crucibles.
Such tanks are generally heated by radiation of a cover placed above the surface of the bath or by immersion of silicon carbide heat exchange fingers.
However, these two types of heating have the disadvantage that, for example in the case of galvanising steel wires, the sheet of wires is accessible only with difficulty and, because of this, there is a restriction to fairly compact sheets of wires.
In addition, the thermal efficiency of installations equipped with these types of heating is generally very poor.
In the case of heating by radiation of a cover placed above the surface of the metallic bath, a major part of the metallic surface is used for heat transfer, so that, in the case of galvanising wires, a large part of the bath cannot be used for the passage of the wires to be galvanised. As a result of this, therefore, the productivity of such installations is somewhat lowered.
Moreover, the high temperature prevailing under the cover accelerates the oxidation of the metallic surface by the steam accumulating above this surface.
Another disadvantage is that this heating method does not allow a uniform distribution of temperature over the transverse cross-section of the bath.
As regards heating using "immersed fingers", the major disadvantages are that the tank is accessible only from one side and also the fact that, as in the aforesaid method of heating by radiation, the temperature is again not uniform over the transverse cross-section of the bath.
Furthermore, the level of the metallic bath must be lowered when a finger is being changed, because the hydrostatic pressure of the molten metal has to be overcome.
Finally, the fingers are sensitive to corrosion at the air-metal interface and it is impossible to start up the heating with these fingers immersed.
Documents FR-A-1 268 223, AU-A-O 544 531 and DE-B-1 133 209 relate to installations with fingers, bells or heating plungers which penetrate to a relatively substantial depth in the baths of metal being heated and which consequently exhibit the disadvantages already mentioned above.